1. Field of the Invention
The present invention relates to a method of and a apparatus for compensating mold thickness changes in a toggle type clamping unit included in an injection molding machine and, more particularly, to a method of and a apparatus which enable to compensate mold thickness changes according to the thickness of a mold without requiring the accurate measurement of the position of a link housing included in the toggle type clamping unit
2. Description of the Related Art
Referring to FIG. 5, a toggle type mold clamping system for an injection molding machine has a fixed platen 2 and a movable platen 3. A mold includes a fixed half mold 4a fastened to the fixed platen 2 and a movable half mold 4b fastened to the movable platen 3. The fixed platen 2 is joined to one end of four tie bars 5. The movable platen 3 is supported on the tie bars 5 for forward and backward sliding movement on a base 6. FIG. 5(a) shows the toggle type clamping unit in a state where the mold 4a and 4b of a great thickness are clamped between the fixed platen 2 and the movable platen 3. FIG. 5(b) shows the toggle type clamping unit in a state where the mold 4a and 4b of a small thickness are clamped between the fixed platen 2 and the movable platen 3.
A toggle link mechanism includes a link housing 7, toggle links 8 interconnecting the link housing 7 and the movable platen 3, a cross head 9 and a servomotor 10. Tie bar nuts 11 are mounted on end portions of the tie bars 5 contiguous with the outer sides of the fixed platen 2 and the link housing 7. When the cross head 9 is driven for forward movement by the servomotor 10, the toggle links 8 are turned so as to advance the movable platen 3 for a mold closing operation.
The toggle clamping unit is able to produce a very high mold clamping force when the toggle links 8 are extended straight Therefore, when the fixed half mold 4a and the movable half mold 4b are completely joined together immediately before the toggle links 8 are extended straight and the tie bars 5 are stretched by a tensile load when the toggle links 8 are extended straight, the mold can be clamped by a very high elastic force produced by the tie bars 5 bearing the tensile load.
The mold clamping force F (tf) of the toggle clamping unit is expressed by:
F=(xcex94Lxc2x7Axc2x7E/1000xc2x7L
where (xcex94L is the elongation (cm) of the tie bars, A is the sectional area (cm2) is the sectional area of the tie bars, E is the modulus of direct elasticity (kgf/cm2) of the tie bars, and L is the distance (cm) between the opposite surfaces of the nuts on each tie bar.
When the mold is replaced with another mold having a thickness different from that of the former, the distance L changes and, consequently, mold clamping force changes greatly. Therefore, the elongation (xcex94L (hereinafter referred to as extra elongation) of the tie bars 5 must be adjusted.
Various systems for compensating for mold thickness changes by adjusting the extra elongation in the toggle type clamping unit are known. For example, a system disclosed in JP-A No. 1989-158521 includes a measuring means including an encoder that measures the thickness of a closed mold (i.e., the distance between the respective working surfaces of a fixed plate and a movable plate), and a calculating means for calculating a correction necessary to be added to an extra elongation to produce a fixed mold clamping force on the basis of the measured distance measured by the measuring means, and drives a toggle type clamping mechanism for movement according to the calculated correction.
A method of compensating for mold thickness changes in a toggle type clamping unit disclosed in Japanese Patent No. 2709868 includes a linear scale for measuring the longitudinal displacement of a rear platen corresponding to the link housing 7 shown in FIG. 5, and measures the distance between nuts on each of tie bars, and a displacement of the rear platen indicated on the linear scale when a reference mold having a movable half mold fastened to a movable platen and a fixed half mold fastened to a fixed platen are locked up. Then, the method measures a displacement of the rear platen indicated on the linear scale when another mold is locked up between the movable platen and the fixed platen. The method calculates an extra elongation and an effective length of the tie bars necessary to clamp the mold by a desired mold clamping force on the basis of the measured values, and sets the movable platen at a position corresponding to the effective length of the tie bars.
The prior art system and method must determine a datum point, thereby determining the positional relation between the fixed platen, the movable platen and the link housing accurately, and hence needs expensive measuring means capable of accurate determination of positions.
In mold clamping force adjustment, a very slight error in the determination of a position results in a great change in mold clamping force. On the other hand, mold clamping force is dependent on external factors including the characteristic of the toggle linkage, the rigidity of the mold, thermal expansion and thermal contraction. Accordingly, the adjustment of the initial condition of the toggle type clamping unit takes much time and cannot be accurately achieved because the initial condition of the toggle type mold clamping system is dependent on various conditions.
Accordingly, it is an object of the present invention to provide a method of and apparatus for compensating for mold thickness changes by adjusting initial platen spacing in a toggle type clamping unit included in an injection molding machine, capable of making the toggle type mold clamping system exerts an appropriate mold clamping force stably on a mold without requiring the accurate measurement of the positional relation between a fixed platen, a movable platen and a link housing included in the injection molding machine.
According to a first aspect of the present invention, a method of compensating for mold changes in a toggle type motor-driven clamping unit having a link housing, a movable platen with a movable mold, a fixed platen with a fixed mold, a toggle link mechanism interconnecting the link housing and the movable platen, and tie bars supporting the link housing, said method comprises steps of dividing a mold thickness adjustable range between a position of the link housing for a thick mold of the greatest allowable thickness and that of the same for a thin mold of the smallest allowable thickness into a plurality of sections; assigning extra elongations of the tie bar applied by the toggle link mechanism for desired mold clamping forces to each of the sections of the platen thickness adjustable range;
moving the link housing forward in a state where the toggle link mechanism is locked up; detecting that which sections the link housing is located in when the movable mold contacts with the fixed mold, and; adjusting the position of the link housing so that the tie bars are elongated by the extra elongation for a desired mold clamping force, assigned to the section in which the link housing is positioned.
According to a second aspect of the present invention, apparatus for compensating for mold changes in a toggle type motordriven clamping unit having a link housing, a movable platen with a movable mold, a fixed platen with a fixed mold, a toggle link mechanism interconnecting the link housing and the movable platen, and tie bars supporting the link housing, said apparatus comprises a plurality of switches that provide binary signals specifying a plurality of divisional sections of a mold thickness adjustable range between a position of the link housing for a thick mold of the greatest allowable thickness and that of the same for a thin mold of the smallest allowable thickness; a dog attached to the link housing and capable of operating on-off action with different combinations of the switches in relative to each of the sections of the mold thickness adjustable range, respectively; a section specifying means capable of specifying one of the sections of the mold thickness adjustable range in which the link housing is positioned on the basis of a combination of the binary signals provided by the switches; a storage means storing data table including extra elongations of the tie bar applied by the toggle link mechanism for desired mold clamping forces and assigned to the sections of the mold thickness adjustable range; an adjusting mechanism including adjusting nuts screwed on threaded end portions of tie bars and an adjusting motor for rotating the adjusting nuts to move the link housing; and a control means capable of reading an extra elongation for a desired mold clamping force assigned to the section in which the link housing is positioned from the data table and controlling the adjusting motor to adjust the position of the link housing so that the tie bars may be stretched by the extra elongation read from the data table.